Hydrodesulfurization process was introduced more than 50 years ago in refineries for fuel quality improvement and for reduction of SO2 emissions to atmosphere. A constant catalyst improvement along years has allowed production of ultra-low sulfur fuels for use in transportation (Catal. Today 86 (2003) 211). HDS catalyst active phase still continues the same from then on, Mo(W)S2, Co(Ni) and alumina. Catalytic community has proposed several models for active sites, such as the sandwich model, decoration model, electronic model, etc. (H. Topose, B. S. Clausen, F. E. Massoth, In Hydrotreating Catalysts: Science and Technology; Springer: Berlin, 1996). Models which explain the selectivity for direct hydrogenation and desulfurization mechanisms in HDS have been also proposed by Daage (cap/edge) and Topsoe (edge) depending on active site relationship. Multicomponent catalysts can be currently designed with a proper function balance to respond to technical demands in ultra-low sulfur gasoline or diesel production, particularly for those fuels produced from heavy crude oils with contents of more than 3% of S by weight and 1% of nitrogenated compounds.
Gas oils commonly produce emissions such as nitrogen oxide, sulfur oxide and carbon solid particles in oil industry, particularly in case of fuels based on petroleum intermediate distillates. Government regulations have been more restrictive in recent years with respect to allowed levels of potentially harmful emissions, leading to a need of deeper gas oil desulfurization (HDS) thus meeting worldwide environmental regulations in force (Catalysis Today 153 (2010) 1-68)
Nowadays, many countries around the globe limit allowed sulfur content in gasoline to less than 50 ppm, and in some specific cases such as Germany, France, Denmark and Sweden up to <10 ppm S (Stanislaus et al.). As environmental concerns grow, allowed sulfur content in gasoline soon may be limited in our country to 10 ppm or even lower. According to these limitations, catalysts and processes for fuel production meeting these requirements would be needed.
Catalytic desulfurization generally includes hydrogenation of susceptible compounds such as olefinic compounds which are present in oil fractions (U.S. Pat. No. 0,230,026 A1). Thus for example in case of high-octane catalytic naphtha, olefins are necessary and thus selective modifiers to decrease hydrogenation reactions are required. Generally, the preparation method used for commercial HDS catalysts is by means of support impregnation with Mo, Co or Ni aqueous solutions. Final structure of molybdenum disulfide surface is achieved in the last preparation stage, where oxide precursor is treated with agents containing sulfur (H2S/H2, DMDS, etc). In this way both, size and morphology of MoS2 resulting particles is affected by several factors such as: (i) the election of Mo compounds, (ii) the influence of chelating species, (iii) precursor structure and dispersion, (iv) the nature of the support itself and its modifiers, (v) surface concentration and Mo species dispersion and finally (vi) the conditions of sulfurization procedure (Catal. Today 150 (2010) 196).
Two types of Co(Ni)—Mo—S active phase models are disclosed in literature. Type-I Co(Ni)—Mo—S phase is characterized by a strong interaction with the support due to Mo—Al—O binding formation and because of a lower sulfurization degree of total Mo available. In type-II Co(Ni)—Mo—S phase, particles are fully sulfurated and Van der Waals type forces keep them on support surface, the specific activity per Co atom number in type-II Co—Mo—S phase is considerably higher than the activity reported for type-I phase. Modern catalysts for deep fuel hydroprocessing such as for diesel are very efficient due to high dispersion and high active phase concentrations therein contained (Catal. Today 149 (2010), 19).
There are several intents reported in literature to selectively obtain highly active CoMoS phase catalysts. There is a method among them including the use of chelating agents in impregnation solutions such as nitriloacetic acid, acetylacetonate, phthalocyanine and ethylendiamine (Appl. Surf. Sci 121/122 (1997) 468). Chelating agents are molecules having two or more donor atoms helping to link a metal cation to form a chelate. By adding chelating molecules to the impregnation solution, the preparation of supported catalysts having an equivalent or higher activity than their commercial counterparts supported on —Al2O3 would be in principle possible for gas oil treatment. Chelating molecules such as ethylendiaminotetraacetic acid (EDTA), nitriloacetic acid (NTA), 1,2-cyclohexanediamine-N,N,N′,N″-tetraacetic acid (CyDTA) and ethylendiamine (EN) have been traditionally used. Still since 1986 a patent was issued for use of such ligands (M. S. Thompson, European Patent EP 0,181,035 A2).
Catalyst synthesis method starts with the preparation of aqueous solutions with Co2+, Ni2+ ions and molybdates which are usually added to porous supports, such as silica and -alumina by incipient impregnation or pore filling. Drying then continues at 120-150° C., and then the catalyst precursor material is fired between 400 and 500° C. to remove counter ions by decomposition. This allows obtaining Co, Ni and Mo ions in oxide state strongly anchored to the support. Interaction of catalyst precursors with the support may be prevented by the use of organometallic complexes of these ions. In this case, sulfurization and preparation of type-II Co—Mo—S active phase is possible. Furthermore, it is important to care about the thermodynamic equilibrium between molybdates, Co2+, Ni2+ and chelating ligands as function of pH and about Ni:Mo or Co:Mo ratios in aqueous solutions (Catal. Today 86 (2003) 173).
Fetchin (U.S. Pat. No. 4,409,131, 1983) performs the synthesis of NiMo and CoMo catalysts supported on Al2O3. For catalyst synthesis in a first stage, cobalt citrate ammoniacal solutions were prepared from citric acid and CoCO3. This solution was heated until boiling point and then cooled. Ammonium hydroxide was added to the resulting solution and diluted to add HMA. This solution was impregnated on Al2O3 support.
Rinaldi et al. (Appl. Catal. A: General 360 (2009) 130) studied the effect of citric acid on CoMo/B2O3/Al2O3 catalysts. Catalysts are synthesized by simultaneously impregnating the B2O3/Al2O3 support with HMA, AC and cobalt nitrate.
Wu et al. (U.S. Pat. No. 0,321,320, 2009) prepared NiMo and CoMo catalysts supported on Al2O3, prepared from Ni(Co) and Mo salts, in addition to an organic acid such as citric acid or urea. Catalysts so prepared showed lower olefin saturation compared to a reference CoMo catalyst.
Ebel et al (U.S. Pat. No. 4,120,826, 1978) synthesized CoMoP/Al2O3 catalysts using cobalt nitrite, H3PO4 and MoO3 as active phase precursors. The phase was impregnated by incipient wetting, and catalysts were dried at 120° C. for 30 min and fired at 538° C. for 1 hour.
Gabrielov et al (U.S. Pat. No. 6,281,158 B1, 2001) synthesized NiCoMoP/Al2O3 catalysts using different active phase precursors. As to Ni precursors, NiCO3 and NiO were used; as to Mo precursors, phosphomolybdic acid, (NH4)2MO2O7, and MoO3 were used while H3PO4 was used as P source. As to Co precursor, CoCO3 was used.
Klimov et al., Catal. Today 150 (2010), 196) introduced a method for catalyst synthesis based on application of Co—Mo labile complexes in aqueous solution with a 2 to 3 Mo atoms per Co atom ratio. The procedure to fix active species on support surface prevented a bimetallic complex decomposition. Bimetallic complexes are synthesized from several ammoniacal Mo salts. By using [Mo4O11(C6H5O7)2]4− as Mo precursor salt (synthesized from ammonium heptamolybdate, HMA, and citric acid), cobalt acetate (Co(CH3COO)2.4H2O) was used as Co precursor, keeping a Co:Mo stoichiometric ratio of 1:2. The impregnated support with the prepared solution was dried in air at 110° C. Catalyst sulfuration was made at 400° C. The catalyst thus prepared was more active than a reference CoMoP/Al2O3 industrial catalyst and other catalysts prepared by the authors.
Pashigreva et al (Catal. Today 149 (2010), 19) report the use of catalysts synthesized from the same precursors used by Klimov et al. (2010 a and b), but modifying the support impregnation method and catalyst sulfuration. Active phase was synthesized from ammonium heptamolybdate, citric acid and cobalt acetate. A higher catalytic performance was also observed in this sample, compared to an industrial CoMo sample which was used as reference.
U.S. Pat. No. 7,618,916 B2 reveals a process for production of a hydrotreating catalyst by a single method, capable of performing an ultra-deep hydrodesulfurization of sulfur compounds present in gas oil without using severe operational conditions. The process comprises impregnation of an inorganic oxide support with metal compounds of Group 6 and Group 8 in the periodic table, an organic acid and phosphoric acid, followed by drying.
Ding et al (Catal. Today 125 (2007) 229) synthesized NiW catalysts supported on Y-type alumina-zeolite composed materials and they were evaluated in light cyclic oil hydrotreating reactions. Supports were prepared by mixing zeolites, alumina (22 or 37% by weight) and a peptidized alumina binder (20% by weight). Materials were firstly dried at 120° C. and then fired at 550° C. for 5 h. Ammonium metatungstate and nickel nitrate were used as active phase precursors. Zeolite final content in catalysts was 15 or 28% for different synthesized materials.
Catalysts used in hydrocracking are of bifunctional type, conjugating acidic with hydrotreating function. Conventional catalysts for catalytic hydrocracking mostly consist of weakly acidic substrates such as amorphous silica-alumina. Many catalysts in hydrocracking market have a silica-alumina base combined with metals from Groups 6 or 8 in the periodic table. Catalysts comprising Y-zeolite with a FAU type or beta-type structure have a catalytic activity higher than those of amorphous silica-alumina in addition to a higher selectivity towards light products. For example, HY-zeolite which is widely used as important component in hydrocracking bifunctional catalysts catalyzes heavy fraction cracking due to its high acidity (Appl. Catal. A: Gen. 344 (2008) 187). It has been proposed that an increase in HDS activity by introducing HY-zeolite might be related to an increase in acidity in mixed supports. Bronsted acidity is capable of catalyzing hydrocracking, isomerization and hydrogenation, which are generally involved in a typical HDS process of petroleum fractions. Addition of less than 10% by weight of zeolite in a CoMo supported on Al2O3 was also shown to be capable of increasing HDS activity up to 40% compared to a conventional CoMo/Al2O3 catalyst when a direct distillation gas oil was evaluated (1.38% by weight of S) (Catal. Today 35 (1997) 45).
Y-zeolite which is used for catalytic cracking processes is produced by a modification of commercially available NaY-zeolite. This process makes possible the modification of stable, ultra-stable zeolites as well as dealuminated zeolites. Preferably, this modification is performed by a combination of three types of operations: (i) hydrothermal treatment, (ii) ionic exchange and (iii) acidic attack. Hydrothermal treatment is perfectly defined by joining operational variables, such as temperature, duration, total pressure and steam partial pressure, with this treatment is possible to extract aluminum from zeolite structure (U.S. Pat. No. 4,277,373). In practice, small zeolite catalyst particles may not be used directly since the powdered material is hard to manage and will cause a pressure drop problem in a packed bed reactor. Therefore, zeolites are normally mixed with inorganic oxides using a binder and the resulting mixture may be extruded with certain shape and size such as reported in literature (Catal. Today 116 (2006) 469), (Appl. Catal. A: Gen. 319 (2007) 25), (Energy Fuels 24 (2010) 796) (Catal. Today 98 (2004) 201).
Dai et al (U.S. Pat. No. 5,308,472, 1994) show a hydrocracking process using Ni(Co)MoP catalyst supported on alumina and silica-alumina with a percentage content by weight of HY-type dealuminated zeolite between 5 and 35%. Ammonium heptamolybdate and nickel nitrate were used as active phase precursors.
Duan et al (J., Catal. Today 175 (2011) 485) synthesized NiMo catalysts supported on alumina-beta zeolite. The mixed support was obtained by mechanical mixture and zeolite content therein varied between 8 and 40% by weight. With a catalyst with a content of 32% by weight of beta zeolite, a removal of 99.4% of S was obtained for a diesel stock with an initial S concentration of 1.3 mg·g−1. Higher zeolite contents favor cracking and coke deposition. Nickel nitrate and ammonium heptamolybdate were used as active phase precursors.
A number of groups have studied the effect of nano and micro zeolites in hydroprocessing catalysts. Ding et al., reported recently the effect of beta zeolite particle size in HDS, HDN and HDA activities used in light cyclic oil hydrotreating with NiMo and NiW catalysts supported on beta-type nano and micro zeolites (Appl. Catal. A: Gen. 353 (2009) 17). Authors reported that there were no significant differences in pore structure, crystalline phases and interactions between metals and supports in two catalysts. They also showed similar activities in light cyclic oil (ACL) HDS and HDN. However, the catalytic behavior in HDS and HDN of a NiW/Al2O3 catalyst with nano-zeolite, was higher than the catalyst prepared with micro zeolite. Another work studied the effect of nano and micro Y-zeolite in HDS and HDN activities with a catalytic cracking diesel in a fluidized bed reactor, using NiMo/Al2O3 mixed catalysts (J. Natural Gas Chem. 20 (2011) 411. The catalyst with nano Y-zeolite showed higher pore average diameters, higher pore volume, a lower amount and less strength of acidic sites, an easier reduction of metal phases, shorter MoS2 films, as well as higher film stack compared to a catalyst prepared with micro Y-zeolite.
Yin et al. (J. Natural Gas Chem. 20 (2011) 441) prepared NiMo/Al2O3 catalysts with type micro- and nano-zeolite and (10% by weight in both cases). Supports were obtained by means of a zeolite mechanical mixture with alumina support. Catalysts were synthesized by co-impregnation of Ni2(OH)2CO3 and MoO3 aqueous solutions. Catalysts with nano-zeolite were more active in HDS and HDN reactions by factors of 3.5 and 2.7, respectively, compared to other samples.